1. Field of the Invention
The present invention relates to a dispersion-compensating system which compensates for dispersion in a wide signal wavelength band.
2. Related Background Art
An optical transmission system employing a WDM (Wavelength Division Multiplexing) scheme transmits a multitude of wavelengths of optical signals in a 1.55-μm wavelength band by way of an optical fiber transmission line network and can carry out large-capacity, high-speed communications. This optical transmission system comprises not only an optical fiber transmission line which is a transmission medium for the optical signals, but also an optical amplifier for collectively amplifying the multitude of wavelengths of optical signals, and the like. In order to enable further larger-capacity/higher-speed communications in such WDM communications, various kinds of research and development have been under way.
As for the optical transmission line, reducing its dispersion and dispersion slope has been an important subject for study. Namely, if the optical transmission line has a dispersion in the wavelength band of optical signals, then their waveforms may be deformed to cause reception degradation when they reach a receiving station by way of the optical transmission line after being sent out from a transmitting station, since each optical signal has a certain bandwidth even though it is considered monochromatic. Therefore, it is desirable that the dispersion of optical transmission line be as small as possible in the signal wavelength band. Also, for larger-capacity communications, it is desirable that the dispersion of the optical transmission line be smaller in a signal wavelength band which is as wide as possible, whereby it is desirable that the dispersion slope of the optical transmission line be as small as possible.
Hence, a dispersion-compensating optical fiber has been connected to a single-mode optical fiber which is usually used for the optical transmission line, so as to reduce the dispersion of the whole optical transmission line. Namely, while the single-mode optical fiber has a positive dispersion in the 1.55-μm wavelength band, the dispersion-compensating optical fiber has a negative dispersion in this wavelength band, whereby an optical transmission line in which the single-mode optical fiber and the dispersion-compensating optical fiber are connected to each other at an appropriate length ratio reduces its dispersion as a whole.
On the other hand, since optical fiber amplifiers which can amplify optical signals in a 1.58-μm wavelength band have been developed, it has been contemplated carrying out optical communications with a further larger capacity by using not only optical signals in the 1.55-μm wavelength band (C band; 1520 nm to 1565 nm) but also those in the 1.58-μm wavelength band (L band; 1565 nm to 1620 nm).